
Adding webkitVisibilityState to webkit document externs.

whitepaper: http://code.google.com/chrome/whitepapers/pagevisibility.html

R=acleung
DELTA=5  (5 added, 0 deleted, 0 changed)


Revision created by MOE tool push_codebase.
MOE_MIGRATION=4337


ajukraine/closure-compiler,ajukraine/closure-compiler